The uvrD,E, recL locus which maps near minute 85 on the chromosome of Escherichia coli will be studied near by localized mutagenesis. A phage lambda or a plasmid containing this locus will also be developed. Mutator mutations that resulted from the insertion of the transposable element Tn5 have been found at the mutS and mutL loci. Together with a deletion of mutH, they will be used to find suppressors of mutator mutations. Loci closely linked to the ast mutator, an allele of mutS, dominantly block the growth of bacteriophages lambda and Pl. The nature of the blocks will be examined.